diff --git a/library/include/mobilisis/calculator_functions.h b/library/include/mobilisis/calculator_functions.h
index bdf2f3f..413429b 100644
--- a/library/include/mobilisis/calculator_functions.h
+++ b/library/include/mobilisis/calculator_functions.h
@@ -32,18 +32,20 @@ public:
     // Daily ticket
     QDateTime GetDailyTicketDuration(Configuration* cfg, const QDateTime start_datetime, uint8_t payment_option, bool carry_over);
 
+    //
+    // helper function to find time steps for a tariff with PaymentMethod::Steps
+    // (e.g. Schoenau/Koenigsee)
+    //
+    QList<int> GetTimeSteps(Configuration *cfg) const;
 
 private:
     // Introduced for PaymentMethod::Steps (e.g. Schoenau)
     // For tariff of following structure: only steps, no special days, nonstop.
-    uint32_t GetCostFromDuration(Configuration const* cfg, QDateTime const &start, quint64 durationMinutes, uint8_t paymentMethod = PaymentMethod::Steps);
-    uint32_t GetCostFromDuration(Configuration const* cfg, QDateTime const &start, QDateTime const &end, uint8_t paymentMethod = PaymentMethod::Steps);
+    uint32_t GetCostFromDuration(Configuration *cfg, QDateTime const &start, quint64 durationMinutes) const;
+    uint32_t GetCostFromDuration(Configuration *cfg, QDateTime const &start, QDateTime const &end) const;
 
 
     //
-    // helper functions for PaymentMethod::Steps
-    //
-    QList<int> GetTimeSteps(Configuration const *cfg, int paymentMethod);
-    //
-    uint32_t GetPriceForTimeStep(Configuration const *cfg, uint8_t paymentMethod, int timeStep);
+    uint32_t GetPriceForTimeStep(Configuration *cfg, int timeStep) const;
+    uint32_t GetDurationForPrice(Configuration *cfg, int price) const;
 };